The present invention relates to a photovoltaic power generating structure using solar cell modules, and more particularly, it relates to a photovoltaic power generating structure which can be formed as a photovoltaic power generating roof having a thermal insulating space portion not communicating with the air on the back surface of each solar cell module.
In recent years, much attention has been paid to the earth""s environmental problems, and the expectations for solar cells which are clean energy sources are increasing more and more. In this connection, various kinds of photovoltaic power generating systems have been suggested and put to practical use. Usually, the above-mentioned photovoltaic power generating system mainly comprises a plurality of solar cell modules arranged on the roof of a house, a junction box electrically connected to the solar cells, and an inverter.
The above-mentioned solar cell modules can be classified into a module using amorphous silicon-based semiconductors and a module using crystalline silicon-based semiconductors. The solar cell module using the amorphous silicon-based semiconductors fundamentally has a lower power generating efficiency than the solar cell module using the crystalline silicon-based semiconductors and, moreover, exhibits a phenomenon (the Staebler-Wronski phenomenon) in which, when irradiated with light for a comparatively short period, a generated output is lower than a generated output in an initial state and is thereafter maintained. Accordingly, in order to obtain the same generated output as in the solar cell module using the crystalline silicon semiconductors, a large light receiving area is required.
This phenomenon of deteriorating power, i.e., light degradation, however, can be depressed by keeping photovoltaic elements at a temperature which is higher than the ambient temperature by 20 to 150xc2x0 C. At the same time, once degraded performance is known to be restored, the phenomenon, known as the annealing effect, provides an effect depending on the temperature.
Usually, solar cell modules are arranged as lifted and constantly spaced on roof tiles or roof base members, so that solar cell modules which employ amorphous semiconductors considerably rise in temperature due to irradiation with sunlight. However, they cannot gain the benefit of the annealing effect due to heat radiation from both the upper and lower surfaces of the modules.
To guard against this, there is a method available whereby the surfaces of the solar cell modules opposite the light receiving surfaces are provided with a thermal insulator or a thermal insulating portion, to obtain the annealing effect sufficiently.
For example, Japanese Patent Application Laid-Open No. 7-292908 employs such a roof construction that the surfaces of the solar cell modules opposite the light receiving surfaces are in contact with thermal insulating materials.
Furthermore, Japanese Patent Application Laid-Open No. 4-33340 employs such a roof construction that solar cell modules and a roof surface plate have a thermal insulating space portion formed therebetween.
The prior art photovoltaic power generating structures have the following problems.
In the case of a roof construction where the surfaces of the solar cell modules opposite the light receiving surfaces are in contact with thermal insulating materials as shown in Japanese Patent Application Laid-Open No. 7-292908, cost effectiveness is problematic. That is to say, to obtain the annealing effect sufficiently, it is necessary to employ thermal insulating materials having a low thermal transmittance, thereby increasing the material costs. It is impossible to prevent thermal radiation from the lower surface of the solar cell modules depending on the thermal conductivity and the thickness of the thermal insulating material employed, resulting in insufficient annealing effect. Moreover, additional costs may be incurred to install the thermal insulating material.
While on the other hand, a roof structure wherein a thermal insulating space portion is formed between the solar cell modules and the roof surface plate such as described in Japanese Patent Application Laid-Open No. 4-33340 has the following two problems.
First, there is a problem due to convection within the thermal insulating space portion. Inside the thermal insulating space portion there are gaps for wiring and the like, even when the portion seems to be finely partitioned by the retaining clips or the like of the solar cell modules. When the air in the thermal insulating space portion is warmed, the air is circulated by convection to the ridge side, resulting in different temperatures in the respective solar cell modules, which leads to fluctuations in total power as a photovoltaic power generating system, as well as light degradation of each solar cell module and nonuniformity in the annealing effect.
Second, there is a problem due to wind. The thermal insulating space portion is in communication with the open air, so that the solar cell modules are subject to upward dashing wind. In order to prevent the solar cell modules from being blown even with strong wind, the solar cell modules must be strengthened greatly.
In view of these problems, an object of the present invention is to provide a photovoltaic power generating structure that is easy to construct, inexpensive to install, able to maintain a good power level as a photovoltaic power generating system, excellent in environmental resistance, and hence improved in fire-resistant properties.
To achieve the above-mentioned objects, a photovoltaic power generating structure according to the present invention comprises a base member on which a solar cell module is arranged in such a way that there is provided a closed space portion interrupted in communication with (i.e., not communicating with or cut off from) the air, between the base member and the solar cell modules. In this case, there exists air or the like in the closed space portion. In the specification and claims, the closed space portion will be sometimes referred to as the xe2x80x9cthermal insulating space portionxe2x80x9d. However, it is not necessary that the thermal insulating space portion completely insulates heat, as long as it has at least substantially the same thermal insulating performance as that of the air.
In the constitution of the solar cell structure, it is preferred that an underlaying material is provided on the base member.
Furthermore, it is preferable that the closed space portion should be divided into a plurality of partitions by a convection controlling portion and that each partitioned space portion is interrupted in communication with each other.
The convection controlling portion should preferably be arranged in the direction of the purlin so that each space portion may be interrupted in communication in the direction of flow of the roof (i.e., the direction of slope of the roof, or the direction perpendicular to the direction of the purlin; hereinafter sometimes referred to as the xe2x80x9cflow directionxe2x80x9d) with each other.
Moreover, the convection controlling portion should preferably be formed of a noncombustible material.
Alternatively, the convection controlling portion may serve also as a retaining clip to fix the solar cell modules.
Furthermore, the solar cell modules should preferably be of the lateral roofing type so that each space portion may be interrupted in communication in the flow direction with each other.
The underlaying material should preferably be a material having a softening point of 40xc2x0 C. to 90xc2x0 C.
Moreover, the underlaying material should preferably be excellent in thermal insulation performance.
In addition, the solar cell modules should preferably be formed into roofing material integrated type solar cell modules.
Further, it is preferred that the photovoltaic power generating structure comprises a non-power-generating roof portion in addition to the solar cell module.
In addition, the solar cell modules should preferably be formed of amorphous silicon-based semiconductors.
Also, the solar cell modules should preferably be connected to an inverter which controls the power of the modules or to a junction box.